Defrost is the default setting, if there's a ducting problem air doors inside the duct are controlled by vacuum cylinders. if you loose vacuum, the actuators go to the default position, and air comes out the defrost. begin by searching under the dash on your back with good lighting, inspect vacuum tubes and connections at actuators near the firewall also inspect under the hood near firewall all vacuum lines and check vacuum cans for corrosion or leaking.
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